In this file photo, Pope John Paul II smiles before addressing the faithful assembled for the traditional Sunday noon blessing at the Pope's summer residence of Castel Gandolfo, near Rome, Sunday August 13, 1995

The historic canonization of two popes is taking place today in St Peter's Square.

Large crowds turned out to see the twin canonisation of two popes - Pope John Paul II, who led the Church from 1978 to his death in 2005, and Pope John XXIII, who held the papacy from 1958 to 1963.

Over a million people were expected to flock to St Peter's Square in Rome on Sunday to witness the ceremony, which was being led by Pope Francis and his predecessor Emeritus Pope Benedict XVI.

Only one miracle out of the two required to be canonised has been performed in the name of Pope John XXIII, but his life was deemed holy enough by Pope Francis, who waived the second miracle.

Two miracles are credited in Pope John Paul II's name. A French nun was cured of Parkinson's disease, and a woman from Costa Rica was cured of a brain aneurysm.

The ceremony is being called "a completely unprecedented event in the history of the Church" as four popes are "symbolically united" for the first time.

The three-hour ceremony on Sunday began at 10am CEST , but broadcast began at 9.30am CEST/ 3.30am ET / 12:30am PT.

Up to 1,200,000 images of the two popes are expected to be transmitted on screens around Rome during the event.

Viewers will also be available to watch the historic event live in 3D movie theatres. Cinemas in Europe, the US and Latin America have been holding special 3D screenings for the spectacular event on the big screen.

This was to allow audiences around the world to feel "that they are actually there in St Peter's Square, fully immersed in the event...in a way that would once have been unthinkable", organisers say.